Many digital control circuits require operator inputs to specify certain parameters. Often such inputs are provided through the use of a keyboard together with interface circuitry. In other cases it is desirable to enable the input of an analog variable. The input of an analog variable is typically easier for an unsophisticated operator to understand. There have been provided in the prior art potentiometers to enable the input of operator specified analog variables. The operator inputs the desired variable by turning the potentiometer to the position of an accompaning scale.
A potentiometer is an electronic component having an electrical resistance which varies with its operator set point. Typically a digital electronic apparatus does not measure the variable resistance of the potentiometer directly. Most typically the variable resistance of the potentiometer is measured by measuring the time constant of an RC circuit in which this variable resistance of the potentiometer is the major contributor to the resistance. This time constant can be measured directly by timing the period needed for the voltage across the capacitor of the RC circuit to charge to a predetermined voltage through the variable resistance of the potentiometer. Alternatively the time constant can be measured indirectly by measuring the frequency of a variable frequency oscillator whose frequency is set by the RC circuit.
The measurement of the variable resistance of the potentiometer in this manner is advantageous from a cost stand point. The measurement of the time constant of an RC circuit, either directly or via measurement of the frequency of a variable frequency oscillator, can be performed using simple timing circuits which are adaptable for use with the typical microprocessor employed in such a digital electronic apparatus. By contrast the direct measurement of the variable resistance of the potentiometer would require the use of some sort of analog to digital conversion device which would be more expensive.
There are problems with use of such a technique for measurement of the variable resistance of a potentiometer. Firstly, the RC time constant is equally sensitive to any variations in the capacitance. This leads to inaccuracies because the typical inexpensive capacitors employed in these digital electronic apparatuses are neither stable or accurately calibrated. Secondly, the time measured by the RC time constant is also sensitive to the supply voltage. In a typical digital electonic apparatus of this type the power supply regulation is poor. Thus there is a need for an inexpensive technique to obtain an accurate measure of the variable resistance of a potentiometer that is relatively insensitive to variations in capacitance and supply voltage.